Internal exams are actually less accurate because there are glands in the vagina that take the water out of the mucus when you're infertile. So when it goes from inside to outside it passes over these glands. If you check internally, that process is interrupted and you can think you have more fertile mucus days than you actually do.
You can keep it really simple. Pay attention to how wet you feel when you walk around during the day - no need to touch the mucus or get it from inside. When you feel wet and slippery during the day, it's time to TTC.